Mobile speed cameras are being used to slow motorists using the Second Severn Crossing after a series of high-speed accidents in recent years.
Several accidents have occurred at the tolls
It is hoped that the cameras, fitted inside special vans, will encourage traffic approaching the toll booths on the westbound carriageway to slow.
The introduction of the cameras follows a 50mph speed limit imposed by the Welsh Assembly Government.
In the last three years, there have been 13 collisions at the toll booth.
These collisions have resulted in people being injured and included top speeds of more than 120mph on the approach.
In August, the assembly introduced a lower speed limit on the bridge, which opened in 1996 to ease the pressure on the original crossing.
Health and safety officials at the toll booths had complained about the high numbers of speeding vehicles, and the vans will park on a flyover above the approach to the bridge.
"The majority of accidents have occurred because motorists fail to slow down on the approach to the toll booth and then collide with stationery vehicles, injuring innocent, law-abiding motorists," said John Rowling, manager of the Safety Camera Partnership which is responsible for camera enforcement in the Dyfed-Powys, South Wales and Gwent Police force areas.
The cameras are fitted inside special vans
"Officials at the toll booth have also expressed concerns for the safety of workers because of the speeds driven," he added.
Superintendent Nigel Russell, head of roads policing for Gwent Police, said: "We have been working closely with the Highways Agency and Welsh Assembly Government to ensure that the reduced speed limit and enforcement are clearly signed to give motorists adequate opportunities to reduce their speed.
"Mobile camera enforcement of the 50mph speed limit will be undertaken and the enforcement vehicle will be clearly visible.
"Motorists are asked to comply with the new limit which has been introduced for the safety of personnel working at the tolls and other road users," he added.